The present invention relates generally to communication systems employing amplification devices. More particularly, the invention pertains to a second order predistortion circuit for coupling between an amplifier and a laser transmitter to minimize the second order distortion output by the laser transmitter.
Amplifiers are widely used in many types of communication applications. For certain communication systems, such as optical communication systems, the amplifier is coupled with a laser transmitter, which generates the optical communication signal. As the need for more precise and reliable communication systems increases, it has become imperative to minimize distortions and achieve a linear frequency response.
Directly modulating the analog intensity of a distributed feedback (DFB) laser is widely used to transmit analog signals, (such as sound or video signals and data), on optical fibers over a long distance. Such amplitude modulation signal typically suffers from nonlinearity of the optical source. DFB lasers are limited primarily by second order distortion.
Laser nonlinearities limit the optical modulation depth M that can be used in the laser. Since the carrier-to-noise ratio of the signal is proportional to the square of the optical modulation depth M, by reducing second order distortion products, the optical modulation depth M can be increased, thus greatly improving system dynamic range.
Referring to FIG. 1, a common prior art method of using a standard RF push-pull amplifier to drive a laser transmitter is shown. A signal is input into the RF amplifier 10 which is connected to a laser transmitter via a balun 12. A balun is a type of transmission line transformer (BALanced-UNbalanced) which allows for the transition between a unbalanced circuit and a balanced circuit and permits impedance matching. In FIG. 1, one leg of the balun 12 is connected to ground, while the other leg of the balun is output to the laser transmitter or other predistortion generating circuits. When an RF amplifier is used to drive a laser transmitter as shown in FIG. 1 over a broad frequency range of input signals, the laser output may be distorted in a non-linear fashion over the frequency range. This non-linear distortion, if not corrected, will degrade the signal performance transmitted by the laser as the output becomes less predictable.
Prior art solutions require the use of numerous complex distortion circuits to correct for second and third order distortion over a broad frequency range. Each distortion circuit corrects a limited portion of the broad frequency range to be transmitted by the laser. For example, U.S. Pat. No. 5,523,716 (Grebliunas) discloses an in-line third order predistortion circuit for satellite applications. Because of the different frequency ranges, bandwidths and power ranges, this design is not appropriate for CATV applications. The power in a satellite applications is much greater than for CATV applications. Accordingly, the diodes used in a satellite application need not be biased. In contrast, for CATV applications, the diodes must be forward biased.
U.S. Pat. No. 5,119,392 (Childs) discloses an inline second order predistortion circuit for use with a laser diode. A field effect transistor (FET) biased for square law operation generates the predistortions. Due to field and doping-dependent variations in carrier mobility, the exact exponent N that can be achieved with a FET varies from between 1.0 and 2.7. The deviation of the exponent N from an ideal number, (i.e., 2.0), causes third order distortion. The difficulties in achieving an ideal exponent N equal to 2, and a good RF frequency response across the entire frequency band by using single stage FET amplifiers limits the performance of this predistortion circuit.
Likewise, U.S. Pat. No. 5,600,472 (Uesaka) and U.S. Pat. No. 5,798,854 (Blauvelt et al.) also generally disclose forward bias diodes used for inline second order or third order predistortion circuits.
Most prior art distortion circuits also require complementary temperature correction circuits for proper operation over a wide range of temperatures. However, each predistortion circuit that is introduced creates additional distortions and losses that degrade the overall performance of the laser transmitter.
Accordingly, it is advantageous to minimize the number of pre- or post-distortion circuits which are utilized to correct for the distortion of the RF amplifier and the laser transmitter.